Studies on replication of plasmid ColEl have been continued. RNA polymerase, ribonuclease H and DNA polymerase I have been found to be essential and sufficient for initiation of DNA replication. RNA polymerase makes precursors of the primer for DNA replication and ribonuclease H cleaves the precursors at specific sites to create the primer for DNA replication that has 555 nucleatides in length. Nucleatide sequences of the regions containing the origin of replication of ColEl related plasmids, pl5A and RSF 1030, have been determined. Structural organizations of the origin regions of these plasmids are quite similar but significant differences in the nucleatide sequences have also been noticed. A mechanism that regulates primer formation has been found.